1. Field of the Invention
The present invention relates to a support member for semiconductor device elements and to a method for reducing instabilities in the operation of the semiconductor device elements due to mobile ions in the support member.
2. Description of the Related Art
When mobile ions, such as sodium ions, are present in a support member that supports a semiconductor device element, operation of the semiconductor device element may become unstable if the mobile ions migrate while the semiconductor device element is being operated. The following background patent documents describe methods for alleviating the operation instability of semiconductor device elements caused by movements of mobile ions in support members. These background technologies are described with reference to the following two patent documents.
Japanese Laid-open Patent Application HEI 7-159809 proposes a technology for stably operating a semiconductor element, or device, on a support member in which, when silicate glass containing mobile ions is used as a support member base, a high density silicon nitride layer is formed on the silicate glass to prevent infiltration of mobile ions from the support member base to the semiconductor device element. Additionally, a phosphorous silica glass film is formed on the silicon nitride layer to thereby block, trap, or fix the mobile ions with the phosphorous silica glass and reduce operation instability due to mobile ions.
Further, Japanese Laid-open Patent Application HEI 5-85777 proposes a technology for stably operating a thin film transistor on a support member. A phosphorsilicate glass film is grown to block mobile ions and thereby reduce operation instability due to movement of mobile ions. Additionally, a silicon oxide film that does not contain phosphorous or boron is formed on the phosphorsilicate glass film.
However, according to the method described in Japanese Laid-open Patent Application HEI 7-159809, a silicon nitride film and a phosphorous silica glass, both of which are electrically insulating materials, are used as support members for supporting semiconductor device elements. In other words, electrically insulating films are used as supporting members in order to block the mobile ions and thereby reduce operation instability due to mobile ions traveling from the support member to the semiconductor device elements. Therefore, there is a problem in that constituent device elements of an electronic circuit that require conductive films cannot be realized using the support members of the HEI 7-159809 patent application. For example, an electric shield film cannot be formed with insulating materials, such as a silicon nitride film or a phosphorous silica glass. Therefore, additional wiring, i.e. conductive, layers are needed to provide electric shielding. The insulating, ion barrier films of the HEI 7-159809 application also make it difficult to form capacitors using capacitive coupling between semiconductor device elements.
Furthermore, because silicon nitride film and phosphorous silica glass are both transparent, i.e. translucent to light, there is a problem in that, when this technology is used for optical devices, MOS transistor regions that would likely malfunction if exposed to light cannot be blocked, or shielded, from light by these films. For this reason, a separate light blocking region, or film/layer, needs to be formed, which results in an increase in the number of manufacturing steps.
Similarly in Japanese Laid-open Patent Application HEI 5-85777, a phosphosilicate glass film and a silicon oxide film, both of which are electrically insulating materials and translucent to light, are used as the supporting member materials. This presents problems similar to those of Japanese Laid-open Patent Application HEI 7-159809.